1. Field of the Invention
The subject matter disclosed generally relates to structures and methods for fabricating solid state image sensors.
2. Background Information
Photographic equipment such as digital cameras and digital camcorders may contain electronic image sensors that capture light for processing into still or video images. Electronic image sensors typically contain millions of light capturing elements such as photodiodes.
Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, photo sensors are formed in a substrate and arranged in a two-dimensional array. Image sensors typically contain millions of pixels to provide a high-resolution image.
FIG. 1 shows a sectional view of a prior art solid-state image sensor 1 showing adjacent pixels in a CMOS type sensor, reproduced from U.S. Pat. No. 7,119,319. Each pixel has a photoelectric conversion unit 2. Each conversion unit 2 is located adjacent to a transfer electrode 3 that transfers charges to a floating diffusion unit (not shown). The structure includes wires 4 embedded in an insulating layer 5. The sensor typically includes a flattening layer 6 below the color filter 8 to compensate for top surface irregularities due to the wires 4, since a flat surface is essential for conventional color filter formation by lithography. A second flattening layer 10 is provided above the color filter 8 to provide a flat surface for the formation of microlens 9. The total thickness of flattening layers 6 and 10 plus the color filter 8 is approximately 2.0 um.
Light guides 7 are integrated into the sensor to guide light onto the conversion units 2. The light guides 7 are formed of a material such as silicon nitride that has a higher index of refraction than the insulating layer 5. Each light guide 7 has an entrance that is wider than the area adjacent to the conversion units 2. The sensor 1 may also have a color filter 8 and a microlens 9.